1. Field of the Invention
The present invention relates to measuring fluid flow. In particular, the present method relates to measuring fluid flow in vessels by ultrasonic means.
2. Discussion of Background
In industrial piping, chemical and nuclear reactors, and elsewhere, it is often necessary to measure the direction and speed of flow of a liquid, gas or slurry. Ideally, this should be accomplished without the need to intrude upon the process being measured. Probes extending into a process line or reactor will, of necessity, disturb the very flow patterns to be measured. Corrosive conditions, high temperatures or pressures, radioactivity, or a combination of these conditions within a reactor or other system may also place severe restrictions upon the materials from which a sensing probe may be constructed, and may also pose the danger of leakage at the point where the probe is inserted.
Instead of a material probe, any one of several forms of energy may be sent into the system; the subsequent behavior of this energy may reveal information about flow patterns. Most commonly, either ultrasonic waves or laser light are used. Inhomogeneities within the system, such as small bubbles or bits of suspended matter, reflect a small portion of the energy back to the radiation source or to a detector placed nearby. Doppler shifts in the reflected energy, showing up as changes in frequency or wavelength, reveal the velocity of motion of the reflecting objects, and hence (usually) of the bulk liquid; unshifted energy, reflected from stationary structural features, can be ignored.
Laser light has the advantage of being able to be precisely focused so that flow rates may be read in a relatively small volume, with minimal interference from the motion of reflecting particles in other parts of the vessel traversed by the beam. However, not all systems permit laser light to be used; some liquids are cloudy or strongly absorbing, and devices are needed to couple the energy efficiently into and out of the system.
Ultrasonic energy is more widely used, since it requires no window and is not so readily absorbed or scattered; ultrasonic transducers are simply attached to the walls of a vessel or to the surface of a pipe, with or without special fittings. At present, however, no ultrasonic flow-measuring systems seem to exist that have the "point-focus" capability of a laser; hence, ultrasonic measurements are typically average values for relatively large volumes of liquid. Such measurement suffices for laminar flow applications, such as with fluid moving slowly through a smooth walled pipe, but is of little value when flow is turbulent.
A means of focussing and controlling ultrasonic waves, permitting "point-focus" capability, would permit convenient and non-intrusive measurement of flow even under turbulent conditions without the need for installation of optical fibers or ports and regardless of the color or turbidity of the fluid under study.